1. Field of Application
This invention relates to valves for the human heart and more particularly to a prosthetic heart valve.
2. Description of the Prior Art
In the field of heart valves there have been many attempts to produce a valve that is similar or substantially similar to the natural heart valve. The reason for this is that if the replacement valve differs in any substantial way from the natural valve its reaction in the heart muscle might be so detremental as to cause failure of the heart itself. One of the most commonly used valves is known as the Starr-Edwards valve. It includes a circular ring with three vertical struts which project upwardly therefrom and join together to form a cage. A steel or plastic ball having a larger diameter than the opening in the ring is movably entrapped in the cage. The pressure of blood flowing through the ring will move the ball away from the ring and permit blood flow through the valve. The reversal of pressure will cause the ball to be seated once again against the ring and thereby block flow through the valve.
There are many problems with the Starr-Edwards type valve. Firstly, the flow is not axial since it must flow around the ball. Such a flow quite often creates a very serious problem since it tends to cause turbulence (eddy currents) which restrict the flow of blood and may cause blood clots. The presence of blood clots in turn leads to thromboembolisms and to prevent this substantial amounts of anticoagulants must be used.
Such blood clots also tend to form around the circular ring and prevent the ball from properly seating itself to close the valve. This leads to serious leakage and ultimate death to the person having the valve. The energy lost in moving the ball is a substantial strain on the heart; and the ball, when it seats against the ring, tends to crush blood cells. Also the cage member, when it projects into the heart causes an obstruction that tends to scar the opposing wall of the heart during the pumping motion. Moreover the ball will occasionally become entrapped in the open position in the cage because of the blood clots or it may escape entirely from the cage. Finally, there is a constant clicking sound caused by the ball hitting the ring, which tends to disturb the person having the valve. These problems have been substantial and have lead to the death of many persons who have had this type of artificial heart valve.
There has been some work done in attempting to produce a valve which simulates the shape and axial flow characteristics of the natural heart valve, and of course, eliminate the ball. Most of these have been directed to tricuspid or three-lip valves.
One such valve requires the use of certain heart valves from swine. One of the problems with this is that hundreds of pigs have to be slaughtered before one or two perfect valves can be found. Secondly, since it is a tricuspid valve, as will be explained hereinafter, if there is slight damage or shrinkage to one of the cusps the valve will not properly close. Also swine valves are not particularly large and because of mounting problems only about 30% of the area of the ring can be used for the opening in the valve. Thus, much of the internal diameter of the normal heart valve is lost in the use of these valves.
Moreover, tricuspid valves have an inherent problem, in that the three lips must close in exact alignment or the valve will not be properly closed. If one lip is slightly out of line or mis-shapen the other two lips cannot make up for the difference and, therefore, leakage will occur. Likewise, after the valve has been used for a period of time, if one of the cusps becomes slightly inflexible or immobile, tissue will build up on that cusp to further reduce it mobility and thereby result in stenosis or total lack of movement of the cusp. This reduces the size of the opening and thereby the amount of blood flowing through the valve; it can cause turbulence in the flow blood and to improper closure. Failure of the valve will thereafter almost invariably occur. Also these valves can function only with rigid metal rings. These rings damage the heart by pressing on the tissues and creating arythmias. Another serious problem of the rigid ring is a high incidence of perivalvular leaks, because the openings in the natural heart valves normally move and they cannot once a rigid ring is inserted. These artificial heart valves thus have substantial problems. Some more of the problems are discussed in detail in the Journal of Thoracic and Cardiovascular Surgery Vol. 68, No. 3, September, 1974, pages 261 to 269.
There has been some effort to produce a two cusp valve which attempts to simulate the shape and flow of the natural heart valve. However, the principal problems with such a valve (as shown in U.S. Pat. No. 3,739,402) have been the lack of an adequate volume of blood flow through the valve and the inability to obtain complete closure. These problems are caused by the rigidity and inflexibility of the lips and also as aforementioned by a rigid supporting ring. The inflexibility of the lips is caused by the rigid ring and also by the tautness of the membrane, which forms the lips. As a result insufficient amount of blood will flow through the heart and cause a tremendous back pressure that can cause serious damage. Also the restriction on flow can cause clots and other serious side effects. Thus, a flexible ring is most important since flexibility will make the ring conform more closely to the overall movement of the heart muscle and will reduce arythmias and trauma to the heart and permit the heart to contract and expand in a manner more closely to normal.
In the patent it states that the lips can open only to approximately 2 millimeters. If we assume the total diameter of the main ring of the valve has to be about 3 centimeters in diameter (which is about the usual diameter of the valves in the human heart) the overall area of opening would be a slit which was 2 millimeters by 3 centimeters or 0.6 sq. cm. in area. When this is compared to the total area of the opening of the ring, which is (3/2.sup.2) about 7 sq. cm., it becomes apparent that the valve opens only to a small fraction of its total area, about 20%. Thus, only 20% of the amount of blood entering the valve will be able to leave resulting in a tremendous strain on the heart and valve, possibly leading to failure of both. Reduction in flow also caused tremendous back pressure within the heart, which can seriously damage the inside of the heart, and further, the restriction in flow can cause clots and other deleterious side effects. The restriction in flow is also complicated by the narrow shape of the struts. The membrane conforms to this narrow shape so that a narrow channel is formed throughout the valve. It is, therefore, seen that the valve not only has a narrow exit, but has a substantially reduced flow area throughout.
The inflexibility of the lips of the device of said patent cause them to close along a thin narrow line along their leading edge. This closing has two serious problems. By having such a narrow line of closure, the slightest deformity in either lip will prevent complete closing and cause leakage through the valve. Second, since the valve closes only along this narrow line, the valve is often unable to remain closed when subjected to the extreme pressures in the heart. This is especially important when the valve is used in the mitral position in the heart, since the pressure differential across the valve in that position is substantial, and any leakage could lead to cardiac failure.
The flexibility of the ring in certain positions, such as the aortic or mitral valve position, will reduce arythimias and trauma to the heart and cause it to function in a normal manner, since it will contract and expand with the heart muscle.
A heart valve is needed which closely simulates the natural heart valve by having the flexibility to open to substantially the same size as the natural opening and the flexibility to completely close and to remain closed against the force of large pressure differentials.